按条件分组和计数 [英] Group and count with condition

问题描述

我正在尝试将一组文档分组并根据它们的值对它们进行计数:

I'm trying to group a set of documents and count them based on their value:

{ item: "abc1", value: 1 }
{ item: "abc1", value: 1 }
{ item: "abc1", value: 11 }
{ item: "xyz1", value: 2 }

我想按 item 分组并返回 value 比 10 大多少次以及多少次小几倍:

I would like to group by item and get in return a count of how many times the value is bigger than 10 and how many times smaller:

{ item: "abc1", countSmaller: 2, countBigger: 1 }
{ item: "xyz1", countSmaller: 1, countBigger: 0 }

推荐答案

你需要的是 $cond 聚合框架的操作符.获得您想要的东西的一种方法是:

What you need is the $cond operator of aggregation framework. One way to get what you want would be:

db.foo.aggregate([
    {
        $project: {
            item: 1,
            lessThan10: {  // Set to 1 if value < 10
                $cond: [ { $lt: ["$value", 10 ] }, 1, 0]
            },
            moreThan10: {  // Set to 1 if value > 10
                $cond: [ { $gt: [ "$value", 10 ] }, 1, 0]
            }
        }
    },
    {
        $group: {
            _id: "$item",
            countSmaller: { $sum: "$lessThan10" },
            countBigger: { $sum: "$moreThan10" }
        }
    }
])

注意:我假定 value 为数字而不是字符串.

Note: I have assumed value to numeric rather than String.

输出:

{
        "result" : [
                {
                        "_id" : "xyz1",
                        "countSmaller" : 1,
                        "countBigger" : 0
                },
                {
                        "_id" : "abc1",
                        "countSmaller" : 2,
                        "countBigger" : 2
                }
        ],
        "ok" : 1
}  

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